meson: ensure correct version-def.h is used

To build the libgit-version library, Meson first generates version-def.h in the build directory. Then it compiles version.c into a library. During compilation, Meson tells to include both the build directory and the project root directory.

However, when the user previously has compiled Git using Make, they will have a version-def.h file in project root directory as well. Because version-def.h is included in version.c using the #include directive with double quotes, some compilers will look for the header file in the same directory as the source file. This will cause compilation of version.c ran by Meson to include version-def.h previously made by Make, which might be out of date.

Copy version.c to the build directory before compiling it to ensure version-def.h from the build directory is used.